CARLSBAD — The leaders of the Carlsbad Irrigation District voted Tuesday to demand the state of New Mexico shut off groundwater users upstream in the Roswell and Artesia areas to protect Carlsbad-area farmers’ right to Pecos River water.

At its worst, the “priority call” could force many groundwater users — municipalities, farms, dairies and oil rigs — to stop pumping water, at a potential cost of $1 billion to the local economy.

The problem, as viewed from Carlsbad, at the bottom end of the system, is upstream groundwater pumping. The groundwater pumpers are “junior” under the doctrine of prior appropriation – they put in there wells after the downstream users had already put surface water from the Pecos to use on the farms of the Carlsbad Irrigation District. Under the doctrine of prior appropriation, the seniors get their water and the juniors get shut off. But the connection between surface and groundwater is slow and indistinct. The groundwater pumping does reduce river flows, but slowly and over time. That means you can’t just shut off the junior pumpers this year and expect to see the river jump right back. Their effect is already baked into the cake, hard to unwind on short time scales:

State officials have argued that any such effort to curtail pumping would amount to a “futile call.” It takes years for the effects of curtailed groundwater pumping to show up in the river, according to Greg Lewis, Pecos Basin manager for the Interstate Stream Commission. That would mean that a call this year would not likely yield additional water for the Carlsbad farmers in 2013, according to Lewis.